Source : Webster's Revised Unabridged Dictionary (1913)

All fours \All` fours"\ [formerly, {All` four"}.]
   All four legs of a quadruped; or the two legs and two arms of
   a person.

   {To be}, {go}, or {run}, {on all fours} (Fig.), to be on the
      same footing; to correspond (with) exactly; to be alike in
      all the circumstances to be considered. ``This example is
      on all fours with the other.'' ``No simile can go on all
      fours.'' --Macaulay.

Source : WordNet®

on all fours
     adv : on hands and knees; "he got down on all fours to play with
           his grandson"